Currently, the most common way to design an integrated oscillator is to use the ubiquitous ring oscillator circuit. As described below, the ring oscillator cell uses an odd number of CMOS inverters. However, a generated frequency of the typical ring oscillator circuit is strongly dependent not only upon the number of CMOS inverters used in the circuit, but upon a temperature of the integrated circuit itself.
In integrated circuit devices, the resistance of load devices is highly temperature sensitive. Variations in resistance are due primarily to changes in surface carrier mobility with temperature. However, integrated circuit capacitors are essentially temperature invariant. When a time constant generating circuit is constructed on an integrated circuit using a resistive load device and an ordinarily constructed integrated circuit capacitor, the RC time constant varies significantly with changes in temperature. The variation in the time constant based on temperature variation thus creates significant problems in stabilizing the oscillator frequency.
The conventional ring oscillator comprises an odd number of inverter stages serially connected in a ring. In a conventional CMOS transistor ring oscillator, each stage comprises a p-channel transistor and an n-channel transistor pair serially connected between first and second voltage potentials, typically a positive supply voltage and ground. The common terminal of the transistors is an output of the stage and is connected to gates of a subsequent transistor pair. Capacitive means shunts an output terminal to ground.
This circuit configuration makes the frequency of the oscillator dependent not only on the number of stages in the ring, but also on the supply voltage, VCC, for the circuit. As VCC increases, the frequency of oscillation increases. Conversely, as VCC decreases, the frequency of oscillation decreases.
With reference to FIG. 1, a prior art ring oscillator circuit 100 includes a plurality of serially connected inverter circuits 101. Each of the plurality of inverter circuits 101 includes a CMOS transistor pair consisting of a PMOS transistor 103 serially connected to an NMOS transistor 105. Each of the plurality of inverter circuits 101 comprises one stage of the ring oscillator 100. To function, the ring oscillator circuit 100 always contains an odd number of stages. The odd number of stages insures an inherent instability of the ring oscillator circuit 100 whereby each of the stages sequentially change state. A capacitive means 107 is associated with the output of each stage. The capacitive means 107 shunts the output of each stage to ground and provides a delay in the changing of states between sequential stages.
With continued reference to FIG. 1, a skilled artisan immediately recognizes that an application of a “zero” (i.e., ground) to the input of one stage causes conduction of the PMOS transistor 103 in that stage. The conducting state of the PMOS transistor 103 charges the capacitive means 107 to a level of the supply voltage, VCC. The charge on the capacitive means 107 thus provides a bias voltage for both the PMOS transistor 103 and the NMOS transistor 105 in the subsequent stage. In such an arrangement, the discharge of each capacitive means 107 through the NMOS transistor 105 of the preceding stage is dependent on the gate bias voltage, VCC. Accordingly, a frequency of oscillation of the ring oscillator circuit 100 depends not only on the number of stages in the ring but also on the voltage level of the supply voltage VCC. Consequently, as VCC increases in voltage, the frequency of oscillation increases. Conversely, when the voltage level of VCC decreases, the frequency of oscillation decreases due to the reduced discharge of each of the plurality of capacitive means 107.
The graph 200 of FIG. 2 indicates the relationship of frequency as a function of temperature. As temperature increases, the frequency decreases, thus illustrating the high temperature coefficient variation that can occur in integrated circuit oscillator devices.
Another prior art approach (not shown) utilizes a simple RC circuit in which a resistor and capacitor are connected in parallel. The capacitor is charged through the resistor until a specified threshold voltage is reached. When the threshold voltage is reached, the capacitor is discharged to a lower voltage threshold. The capacitor in such a circuit is frequently connected to a comparator in which a comparator output is used as an oscillator reference clock. Such a circuit still suffers from the drawbacks described with reference to FIG. 2.
In another prior art approach, shown with reference to FIG. 3, a bandgap reference-based oscillator circuit 300 includes a bandgap reference cell 301, a current reference source 303, a voltage comparator 305, and a capacitor 307. The bandgap reference cell 301 is known in the art to generate a voltage which has a very low temperature variation coefficient. In the bandgap reference-based oscillator circuit 300, the current reference source 303 generates a reference current. The current, in turn, linearly charges the capacitor 307. The voltage comparator 305 uses the bandgap reference cell 301 as a threshold reference. When a voltage level of the capacitor 307 reaches the bandgap voltage, the capacitor 307 is discharged and the charge/discharge sequence is repeated. The bandgap reference-based oscillator circuit 300 thus achieves a stable output frequency based upon the very low temperature variation coefficient of the bandgap reference cell 301 to generate voltages and currents. However, a major disadvantage of the bandgap reference-based oscillator circuit 300 is that it requires both significant power consumption and silicon area thereby driving up both the cost of fabrication and the cost of operation.
Therefore, what is needed is a simple and inexpensive oscillator circuit. The oscillator circuit will additionally have low power consumption, require little layout area, and not be dependent upon voltage and temperature variations. Such a circuit should be able to be readily integrated with other integrated circuit devices in a fabrication environment.